The slogan "The once and future king" on the kitchen wall was written by Edward (Laurence Fishburne), who was left handed. As such the first couple of words are rubbed slightly by his sleeve. In the final scene we see a blood-spattered slogan, this time written perfectly without the faded first couple of words.
When Jimmy and Cooper are sitting in the coffee shop, Cooper holds up the camera and the lever on the bottom is pulled back. Then, when the shot comes back to them, the lever is forward and Jimmy reaches over and pulls it back.
While John Casey is seated and talking, his wrist watch says the time is 10 minutes to the hour. A few moments later, the watch has skipped back in time to quarter to the hour.
In the opening sequence, Jose puts on his white jacket, buttons it at the neck and the collar is tucked in at the back. During the scene the collar becomes straight without his intervention.
The Christian Slater character violently grabs William Macy between the first and second buttons on the tuxedo shirt with a completely bloodied hand. Later at the ambulance, Macy's perfectly buttoned shirt has a only few minor specks of blood, should be a bloody mess.
The credits include the closing speech detailing the speech as "Robert F. Kennedy's speech, 'On The Mindless Menace of Violence.' The credits say it was delivered in Indianapolis, Indiana on April 5, 1968. This is incorrect; Robert Kennedy gave a speech on Martin Luther King, Jr.'s death in Indianapolis on the previous day but gave the speech presented on the recording at the City Club of Cleveland in Cleveland, Ohio.
In the scene where Samantha and Jack are playing tennis, if you look behind the ivy in the background you can see present day traffic. The movie takes place in 1968.
During a conversation between John Casey and Nelson, there is close-up shot of Nelson: you can see that he is wearing an in-canal hearing aid in his left ear. These were not available in 1968.
The Ambassador Hotel's lobby scene, in which John Casey and Nelson talk, shows Casey wearing a quartz watch. It was June 1968. The world's first commercial quartz wristwatch, the Seiko Astron, would only be introduced in December 1969.
A scene in a men's restroom shows ADA-complaint soap dispensers with low-force plastic valves. These did not exist until 1990.
(at around 21 mins) 4-beam sky-tracker (Pichel) style searchlight beams are shown. These were first brought to market in the 1979-1980.
When Tim starts playing drums with the band, the sound of the cymbal doesn't match what he's playing.
In Samantha's glasses at the pool, you can see very clearly in the reflection the 20x20 white diffusion with crew members either standing or sitting in front of it.
When John wins the chess game and Paul is called over, Paul gives John's opponent advice on how he could have won. Paul couldn't have known this so quickly without observing the game or the board for at least a few minutes.